As is often the case, advancements in any one of these design objectives of this multivariate optimization problem do not necessarily correspond to advances in the others. For example, the energy density and power density of the cell are primarily determined, respectively, by the electrochemical stability window of the electrolyte and the ion transfer kinetics. Aqueous electrolytes typically offer high ionic conductivity, but often suffer from a narrow electrochemical stability window that limits the achievable cell voltage and energy density. By contrast, organic electrolytes provide a wider electrochemical window that enables higher energy densities, but generally have lower ionic conductivity — many organic electrolytes are, moreover, flammable, which can make their safe operation more complicated. One might also consider combining both aqueous and organic systems to reap the advantages of both types of electrolyte; however, the formation of the biphasic interphase between the two types of electrolyte increases the lithium-ion transport impedance, which can severely compromise the overall electrochemical performance of the battery.
Now, Chunsheng Wang and co-workers report on a modification to the solvated structure of Li+ ions to facilitate their transfer in an aqueous–organic biphasic electrolyte. Specifically, the authors used a 12-crown-4 ether (12C4)-based ionophore, with a cavity size complementary to Li+ and a higher binding affinity than water molecules, to selectively coordinate Li+ and exclude water from its primary solvation shell. Coupled with a carefully designed biphasic electrolyte system, Li+ ions, encapsulated by 12C4, were found to efficiently migrate between the two phases. Simulations and electrochemical tests demonstrated a notable reduction in ion transport impedance at the interface of the constructed biphasic electrolyte system, with the electrochemical stability window measured to be 0.0–4.9 V versus Li|Li+ at room temperature.
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